What is the difference between a terrorist and an extremist?

Initial media portrayal of the Oslo attacks strongly insinuated a decidedly “Muslim” element. From The New York Times headline, to statements by BBC and Washington Post. Even President Barack Obama issued the statement, “It’s a reminder that the entire international community holds a stake in preventing this kind of terror from occurring” and “we have to work cooperatively together both on intelligence and in terms of prevention of these kinds of horrible attacks”. Effectively implying that an international terrorist group is responsible.

Now it has been brought to light that the perpetrator was a Norwegian right-wing extremist with a history of anti-Muslim affiliation.

If this is not terrorism by definition, does it logically follow that terrorism is only a function of who perpetrates the terror?

Are terrorist and extremist mutually exclusive or one and the same thing?

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